I would be cautious bleaching your hair and then directly applying hair dye to it after. Bleach is the worst thing you can do to your hair. It blows all of your follicles open, so if you apply the hair dye directly after, the color won't hold as long as it should.

What you should be doing is lifting the color of your hair out first with a kit from Sally's or something and then bleaching it, or it's very likely that going from a dark brown you'll end up with a nasty coppery-orange color, which isn't something you can just apply a pastel color to. You'll need to bleach it after lifting the brown from it (make sure to get toner and work it through afterward to make it nice and bright), and then you should wait a couple of days for your hair to mend before adding your chosen color to it.

If you want a nice pastel color, mix equal parts of conditioner in with your chosen color. Remember that the dye will be darker than how it actually looks on your head, so don't keep adding conditioner to it or it will be useless.

I like the light lavender color on boho girls, so it'll probably look cute! I would also suggest the Manic Panic brand for colors because they are the best.